Here is the AMP image size error in URL Inspection tool:
Upload Featured Image of 1200px Width
To fix this image size error on AMP pages you can upload high quality images as a Featured Image that has a width larger than 1200px.
Fix AMP error on older posts
Another way to solve image size error all older posts or pages that has no Featured Image is to upload and set a default image to be used as a fallback for missing Featured Images.
Schema Premium has this as a feature in Schemas settings.
By defining a default image, you will be able to solve this AMP error.